1. The Field of the Invention
The present invention relates to electronic circuits and more specifically to methods and devices for protecting electronic circuits. In embodiments of the present invention, the protection circuit is preventing reverse current flow from an output of a low drop regulator to a supply (battery) and has good resilience to electromagnetic disturbances on the supply line. The methods and devices of the present invention may for example be used in a harsh environment such as an automotive environment. They can handle transient negative overvoltage pulses without influencing functionality of the electronic circuit during such pulses. They furthermore improve susceptibility of the electronic circuits against electromagnetic disturbances on the supply line.
The protection circuit can be also used if reverse battery protection or mis-wiring protection is required. In embodiments of the present invention it is protecting from damage a circuit if polarity of the supply source is reverted and supply potential (VCC) is below ground.
2. Background of the Invention
Fast development of electronics in the last years with an increasing number of transistors in circuits, increasing clock frequency together with the existence of more mobile communications (GSM, UMTS, . . . ) sets higher and higher requirements for electromagnetic compatibility of electronic devices.
Especially for automotive electronics there are new and more severe requirements defined by new standards. The Bulk Current Injection (BCI) test (defined by ISO 11452, part 4) or transient pulse overvoltage immunity (so-called Schaffner pulses) are only two examples. The slow direct capacitive coupling (DCC) method or inductive coupling clamp (ICC) method pulses are additional examples of standards with more severe requirements (defined by ISO/CD 7637-3:2004).
During the development phase of an electrical circuit, e.g. an ASIC, typically most of the attention is paid to the functional and parametric behavior of the circuit. The EMC behavior is too often neglected during the design phase and has to be corrected after the prototype phase, which increases the costs and prolongs the time to market.
U.S. Pat. No. 4,423,456 describes a protection circuit for protecting an electrical circuit from damage due to polarity reversal of the power supply. In its most elementary form, the protection device comprises a transistor switch adapted for opening the power source to load circuit upon polarity reversal. The transistor is a P-channel enhancement MOSFET of which the source is coupled to the positive terminal of the power supply and the drain is coupled to the positive terminal of the load. If the power supply is applied with proper polarity, the gate of the transistor switch is negative with regard to the source of the transistor switch, and current flows from the source to the drain of the transistor switch. This represents normal operation. Should the power supply polarity be reversed by improper battery insertion, the gate of the transistor switch becomes more positive than the source. This effectively results in an infinite source-to-drain resistance, thereby protecting the load from the polarity reversal.
EP-1357658 describes a protection circuit against voltage transients and polarity reversal. It uses a main protection transistor and switching circuit to control its gate in case of negative voltage on the supply line. A disadvantage of the solution described is that it requires and uses thick gate NMOS devices, which results in a quite high voltage drop over the NMOS switching transistor or a need for a charge pump.